short for Medieval Latin pro bono publico "for the public good;" from pro (prep.) "on behalf of, for" (see pro-) + ablative of bonum "good" (see bene-).
Entries linking to pro bono
pro-
word-forming element meaning "forward, forth, toward the front" (as in proclaim, proceed); "beforehand, in advance" (prohibit, provide); "taking care of" (procure); "in place of, on behalf of" (proconsul, pronoun); from Latin pro (adv., prep.) "on behalf of, in place of, before, for, in exchange for, just as," which also was used as a first element in compounds and had a collateral form por-.
Also in some cases from cognate Greek pro "before, in front of, sooner," which also was used in Greek as a prefix (as in problem). Both the Latin and Greek words are from PIE *pro- (source also of Sanskrit pra- "before, forward, forth;" Gothic faura "before," Old English fore "before, for, on account of," fram "forward, from;" Old Irish roar "enough"), extended form of root *per- (1) "forward," hence "in front of, before, toward, near," etc.
The common modern sense of "in favor of, favoring" (pro-independence, pro-fluoridation, pro-Soviet, etc.) was not in classical Latin and is attested in English from early 19c.
bene-
sometimes beni-, word-forming element meaning "well," from Latin bene (adv.) "well, in the right way, honorably, properly," from PIE *dwenelo-, suffixed (adverbial) form of root *deu- (2) "to do, perform; show favor, revere." Opposed to mal-. From the same source come Latin bonus "good," bellus "handsome, fine, pretty," and possibly beatus "blessed," beare "to make blessed."