semi- – полу-

semi- (словообразовательный элемент, означающий – половина, часть, частично, несовершенный, двойной; из латинского semi- половина
before vowels sem-, word-forming element meaning "half, part, partly; partial, imperfect; twice," from Latin semi- "half," from PIE *semi- "half" (cognates: Sanskrit sami "half," Greek hemi- "half," Old English sam-, Gothic sami- "half").

Old English cognate sam- was used in such compounds as samhal "poor health," literally "half-whole;" samsoden "half-cooked," figuratively "stupid" (compare half-baked); samcucu "half-dead," literally "half-alive;" and the last survivor of the group, sandblind "dim-sighted" (q.v.). Common in Latin (as in semi-gravis "half-drunk," semi-hora "half hour," semi-mortuus "half-dead," semi-nudus "half-naked," semi-vir "half-man, hermaphrodite").

1828:
SEMI – половина; из ήμι

SEMIS – половина; Ήμισυ; у Вейсмана - ήμι

Бикс объясняет из корня «sem» - один из, т.е. «сам»; «сую» - «семя», «земя», «семья», «самец», см. «same», «seem», «semen»