Douglas Harper's Etymology Dictionary
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").\n
\nOld 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"). The Latin-derived form in English has been active in forming native words since 15c.
The Collaborative International Dictionary
Semi- \Sem"i-\ [L. semi; akin to Gr. ???, Skr. s[=a]mi-, AS. s[=a]m-, and prob. to E. same, from the division into two parts of the same size. Cf. Hemi-, Sandelend.] A prefix signifying half, and sometimes partly or imperfectly; as, semiannual, half yearly; semitransparent, imperfectly transparent.
Note: The prefix semi is joined to another word either with the hyphen or without it. In this book the hyphen is omitted except before a capital letter; as, semiacid, semiaquatic, semi-Arian, semiaxis, semicalcareous.
pre. 1 half 2 partial, incomplete 3 somewhat, rather, quasi-
Longman Dictionary of Contemporary English
Usage examples of "semi-".
But Johnson said nothing, responding only for a semi- second with a look of contemptuous ingratitude.