sweeten

   1. to bribe

   Using the common imagery of making something more toothsome:

    Now-a-days ane canna' phraise,

   An' sooth, an' lie, an' sweeten, An' palm, an' sconse. (Lauderdale, 1796 — referring to flattery, bribery, and trickery)

   And in modern use of an improper inducement:

    Construction had been held up by the Pollution Control Board. A $30,000 fee was negotiated, sweetened with the offer of a job. (Evans-Pritchard, 1997)

   A sweetener is such a bribe, not necessarily in cash:

    Giving big commissions, sweeteners, call it bribery if you like... (Lyall, 1980)

   2. (of a public auction) improperly to force up bidding

   Auctioneers' jargon for the practice of purporting to accept spurious or nonexistent bids.

   3. to attempt to improve by deception

   Showbusiness jargon of the practice whereby a producer introduces pre-recorded laughter to give the impression that an audience found a show funnier than in fact they did:

    Producers... devised what they believed was a totally justified method of sweetening a show. (F. Muir, 1997 — they had a comedian tell a vulgar joke, and cut the resultant laughter into another recording)