half is used typically when you say something that you are only half joking/serious about. It can be used after just kidding instead of saying just kidding...kind of. in other words, half is a subsitution for the word kinda. Or it can be used in any statement that is only half true.
however percentages may not be used but fractions are optional. and if you are fully kidding, just say just kidding... not full kidding. it is not acceptable.

half and half is when someone sucks your dick and then your asshole...a little blow job, then a little rim job...half in front, half in back....usually an action performed by a hooker

A mixed drink of lemonade and ice tea. Prefectly complements a chicken box or lake trout. Genuine half and half is only found in southern cities in "ghetto" areas.

Like a hard on, it comes about when some thing is arousing, but not enough to induce a full erection.

A semi-erect penis. (See: Semi, Chub) Fifty percent of a Hard-On. A "bulge in the pants," as it were.

A cheap brand of "all-purpose" smoking tobacco - one of the earliest packaged, branded pipe tobaccos in the U.S. - with the slogan "a cargo of contentment in the bowl of any pipe." A long-standing joke involving the product's name asserts that it is named "half and half" because it is comprised of "half horseshit and half bullshit" (in reality, the mixture is half "bright" and half "burley" tobaccos). Some old-timers speculate that the half and half joke may have given rise to the novelty "horseshit cigarettes" sold in the U.S. in the 1920s-30s (not real horseshit), which featured the slogans "not a fart in a carload," and "try one, you mooching bastard."

Half and half is still sold at most retail outlets carrying tobacco products.

Something definitely not up to par; lacking madly