Old English hilt "hilt, handle of a sword or dagger," from Proto-Germanic *helt (cf. Old Norse hjalt, Old High German helza "hilt," Old Saxon helta "oar handle"), perhaps from PIE *kel- "to strike." Formerly also used in plural in same sense as singular.
Also, up to the hilt. Completely, to the maximum degree, as in The house was mortgaged up to the hilt. This idiom alludes to the handle (hilt) of a sword, the only portion that remains out when the weapon is plunged all the way in. The figurative use of the term was first recorded in 1687.