Theres two meanings:
n1) Old French groign (French groin), Burgundian groigno, Provençal
groing, grong, feminine groingna, Italian grugno, Old Portuguese gruin
< popular Latin *grunnium, < Latin grunnīre to grunt like a swine
n2) originally grynde , grinde of uncertain origin; connection with
grain n.2 is excluded by the early form. The form grine appears in
1530, but did not finally displace grind until the last quarter of the
16th cent., when it underwent corruption into groin , probably through
phonetic association with groin n.1
Prof. Skeat suggests that the original sense may have been a channel
or depression (compare 2), and that the word may be identical with Old
English grynde , recorded only in the sense of ‘abyss’, but
etymologically capable of meaning ‘depression’, ‘valley’ ( < prehist.
*grundjo- , < *grundu- ground n.; compare German grund , used dialect for ‘valley’; also grindle n.1, grindlet n.).
The grunting one seems more like what would happen in a battle.
Upvote for the other answer though!