Yee is an exclamation of excitement (as in 'yee-haw'), or a dialectal variant of ye/you; the interjection is imitative. In current use it is an intj and pronoun meaning "an exclamation of excitement, or a dialectal form of you/ye". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The history is chiefly morphological, explaining how the modern form packages an older base into its present English use. Related English forms include ye and you. For dictionary purposes, the important point is the continuity between the older form, the route into English, and the present lexical sense.