Context:
tortoise
1550s, altered (perhaps by infl. of porpoise) from
M.E. tortuse (late 15c.), tortuce (mid-15c.), tortuge (late 14c.), from
M.L. tortuca (mid-13c.), perhaps from L.L. tartaruchus "of the underworld" (see turtle). Others propose a connection with
L. tortus "twisted," based on the shape of the feet. The classical L. word was testudo, from testa "shell." First record of tortoise shell as a coloring pattern is from 1782.
More:
http://www.etymonline.com/index.php?search=tortoise&searchmode=none