why is it called the "calf" muscle?

Earlier, I inadvertently posted my answer in the "Comments" section.
Please forgive my error.

The English word "calf" for a muscle in the lower leg comes from an
Old Norse word, kálfi. It is related to the Irish Gaelic word "calpa."
Authorities are uncertain on the matter of whether or not, in the
distant past, this word may have shared a common Germanic root with
the other "calf," meaning a young cow.

Thanks to the Vikings who invaded the British isles from the Eighth
Century onwards, the English language contains many words that
originated in Old Norse.

"calf...
The fleshy muscular back part of the human leg between the knee and ankle.

Middle English, from Old Norse k?lfi."

Answers.com: Calf
http://www.answers.com/topic/calf

"Calf...
 Old Norse kálfi, possibly from Irish calpa' possibly akin to CALF1
[meaning a young cow] from its shape."

Wiktionary: Calf
http://en.wiktionary.org/wiki/Calf

"calf  
O.E. cealf 'young cow,' from W.Gmc. *kalbam, perh. from PIE *gelb(h)-,
from base *gel- 'to swell,' hence, 'womb, fetus, young of an animal.'
Elliptical sense of 'leather made from the skin of a calf' is from
1727. Used of icebergs that break off from glaciers from 1818. Calf of
the leg is from O.N. kalfr, source unknown; possibly from the same
Gmc. root."

Etymology Online: Calf
http://www.etymonline.com/index.php?term=calf

"CALF
 Hind part of leg below the knee. Before 1325. 
 Borrowed from Old Icelandic Kalfi
 Related to kalfr
 Possibly akin to calf (a young bull) from its shape.
 ON kalfi, of unknown origin
 Adoption from Irish Gaelic calpa, leg, calf of the leg (conjectured)

--American Heritage Dictionary

The calf of the leg comes from ON kalfi, akin to ON kalfr, a quadruped
calf, itself akin to OE cealf, whence the quadruped calf. OE cealf is
akin also to OHG (and G) kalb, Go kalbo, ON kalfrs. That the basic
idea in IE is 'a swelling of the body' is supportedby Gaul galba, a
fat man, and Sanskrit garbhas, womb. The Gmc ris *kalbh-; the IE r,
*gwolbh-, with var *gwalbh-:Ga calpa, OIr colpa, calf of the leg.

----Partridge, Eric. Origins: A Short Etymological Dictionary ofModern
English. The Macmillan Co. New York, NY. 1958."

California State University, Fullerton: Paper by Colleen Tucker
http://hss.fullerton.edu/comparative/calf.pdf

From the unabridged Oxford English Dictionary:

"Calf...
ON [Old Norse] kálfi of unknown origin; adoption from Ir. Gael.
'calpa' leg, calf of the leg, has been conjectured."

The OED cites 1325 as the date when the earliest usage appeared in
print. Around 1386, Chaucer used the word "calf" in "The Canterbury
Tales."

My Google search strategy:

Google Web Search: calf leg "old norse"
://www.google.com/search?hl=en&q=calf+leg+%22old+norse%22

Google Web Search: calf leg calpa OR kálfi OR kalfi OR kálfr OR kalfr
://www.google.com/search?hl=en&q=calf+leg+calpa+OR+k%C3%A1lfi+OR+kalfi+OR+k%C3%A1lfr+OR+kalfr

I hope this is helpful! If anything is unclear or incomplete, please
request clarification; I'll be glad to offer further assistance before
you rate my answer.

Best regards,
pinkfreud

Thanks for including the various sources of etymology.  That really helps.

The English word "calf" for a muscle in the lower leg comes from an
Old Norse word, kálfi. It is related to the Irish Gaelic word "calpa."
Authorities are uncertain on the matter of whether or not, in the
distant past, this word may have shared a common Germanic root with
the other "calf," meaning a young cow.

Thanks to the Vikings who invaded the British isles from the Eighth
Century onwards, the English language contains many words that
originated in Old Norse.

"calf...
The fleshy muscular back part of the human leg between the knee and ankle.

Middle English, from Old Norse k?lfi."

Answers.com: Calf
http://www.answers.com/topic/calf

"Calf...
 Old Norse kálfi, possibly from Irish calpa' possibly akin to CALF1
[meaning a young cow] from its shape."

Wiktionary: Calf
http://en.wiktionary.org/wiki/Calf

"calf  
O.E. cealf 'young cow,' from W.Gmc. *kalbam, perh. from PIE *gelb(h)-,
from base *gel- 'to swell,' hence, 'womb, fetus, young of an animal.'
Elliptical sense of 'leather made from the skin of a calf' is from
1727. Used of icebergs that break off from glaciers from 1818. Calf of
the leg is from O.N. kalfr, source unknown; possibly from the same
Gmc. root."

Etymology Online: Calf
http://www.etymonline.com/index.php?term=calf

"CALF
 Hind part of leg below the knee. Before 1325. 
 Borrowed from Old Icelandic Kalfi
 Related to kalfr
 Possibly akin to calf (a young bull) from its shape.
 ON kalfi, of unknown origin
 Adoption from Irish Gaelic calpa, leg, calf of the leg (conjectured)

--American Heritage Dictionary

The calf of the leg comes from ON kalfi, akin to ON kalfr, a quadruped
calf, itself akin to OE cealf, whence the quadruped calf. OE cealf is
akin also to OHG (and G) kalb, Go kalbo, ON kalfrs. That the basic
idea in IE is 'a swelling of the body' is supportedby Gaul galba, a
fat man, and Sanskrit garbhas, womb. The Gmc ris *kalbh-; the IE r,
*gwolbh-, with var *gwalbh-:Ga calpa, OIr colpa, calf of the leg.

----Partridge, Eric. Origins: A Short Etymological Dictionary ofModern
English. The Macmillan Co. New York, NY. 1958."

California State University, Fullerton: Paper by Colleen Tucker
http://hss.fullerton.edu/comparative/calf.pdf

From the unabridged Oxford English Dictionary:

"Calf...
ON [Old Norse] kálfi of unknown origin; adoption from Ir. Gael.
'calpa' leg, calf of the leg, has been conjectured."

The OED cites 1325 as the date when the earliest usage appeared in
print. Around 1386, Chaucer used the word "calf" when referring to a
portion of the human leg in "The Canterbury Tales."

My Google search strategy:

Google Web Search: calf leg "old norse"
://www.google.com/search?hl=en&q=calf+leg+%22old+norse%22

Google Web Search: calf leg calpa OR kálfi OR kalfi OR kálfr OR kalfr
://www.google.com/search?hl=en&q=calf+leg+calpa+OR+k%C3%A1lfi+OR+kalfi+OR+k%C3%A1lfr+OR+kalfr

I hope this is helpful! If anything is unclear or incomplete, please
request clarification; I'll be glad to offer further assistance before
you rate my answer.

Best regards,
pinkfreud