[Etymonline :] "unstressed syllable at the beginning of a verse," 1833, Latinized
from Greek anakrousis "a pushing back," of a ship, "backing water,"
from anakrouein "to push back, stop short, check," from ana "back" (see ana-) + krouein "to strike,"
from PIE *kreue- (2) "to push, strike"
In what sense can a pickup note be judged as pushing back or up (the definition of 'anacrusis' in Ancient Greek)? (Nescient of poetry) I do not understand Wikipedia's explanation) of the semantic shift from poetry:
In poetry, a set of extrametrical syllables at the beginning of a verse is said to stand in anacrusis (Ancient Greek: ἀνάκρουσις "pushing up"). The technique is seen Old English poetry, and in lines of iambic pentameter, the technique applies a variation on the typical pentameter line causing it to appear at first glance as trochaic.
[...] The musical term is inferred from the terminology of poetry, where it refers to one or more first but unstressed syllables of a lyrical verse. [...]