abet (v.) Look up abet at Dictionary.com
late 14c., "urge on, incite" (implied in abetting), from Old French abeter "to bait, to harass with dogs," literally "to cause to bite," from a- "to" (see ad-) + beter "to bait." This verb is probably from Frankish or some other Germanic source (perhaps Low Franconian betan "incite," or Old Norse beita "cause to bite"); ultimately from Proto-Germanic *baitjan, from PIE root *bheid- "to split" (see fissure (n.)). Sense of "encourage by aid or approval" is from 1779. Related: Abetted; abetting.
bet Look up bet at Dictionary.com
1590s, as both a verb and noun, in the argot of petty criminals, of unknown origin; probably a shortening of abet or else from obsolete beet "to make good," from Old English bætan "make better, arouse, stimulate," from Proto-Germanic *baitjan, in which case the verb would be the original. The original notion is perhaps "to improve" a contest by wagering on it, or it is from the "bait" sense in abet. Used since 1852 in various American English slang assertions (compare you bet "be assured," 1857). Related: Betting.