Definition of turtle noun from the Oxford Advanced Learner's Dictionary

turtle

noun
ˈtɜːtl
 
; ˈtɜːrtl
 
 
1 (North American English also sea turtle) a large reptile with a hard round shell, that lives in the sea2 (North American English, informal) any reptile with a large shell, for example a tortoise or terrapin
Idioms

turn turtle

(of a boat) to turn over completely while sailing