The word 'sub' is a direct borrowing from Latin, where it served as a preposition and prefix with the core meaning of 'under'. This Latin term is itself derived from the Proto-Indo-European root upo, which carried meanings related to being under or up from under. In English, 'sub' retains its original sense as a productive prefix in words like 'submarine' (under the sea) and 'subway' (an underground way). Through functional shift, the clipped form of nouns like 'submarine' and 'substitute' entered common usage as standalone nouns and verbs, demonstrating a process of linguistic abbreviation where the prefix came to represent the whole.

The word 'sub' is a morpheme itself, functioning as a prefix, noun, or verb. It originates from the Latin preposition 'sub', meaning 'under, below, beneath'.