Pronounced 'IH-dah', it is an acronym for "I'd Do Her." Essentially it is a rating system for hot girls, with a fairly complex yet simple, relatable, and customizable set of paramaters. The idea is that you can talk about hotness of girls in a public place without alerting their suspicions, and it illustrates a better idea of the hotness of a girl than the traditional 1-10 rating system.
IDH: Baseline doable girl. Hot but nothing special.
DIDH: Definitely I'd Do Her. Really hot girl that you'd pounce on given the chance. Boobs poppin out and really hot body.
IDH + n B: where B means beers and 'n' is a variable of amount of beers. So "IDH plus 3 B" would be a girl who would be doable after three beers.
IDH +/- n Y: Where Y is a number of years. So if you see a girl who would be hot once they hit 18, you'd say "IDH plus 4 Y". Likewise, an older woman that was probably really hot 15 years ago but is now just a cougar would be "IDH minus 15 Y"
TIDH/Badoobadaboob: "Tits I'd Do her" - basically this IDH is mainly hot because she has a really nice, unignorable pair of punching bags.
IFH: I'd Fuck Her. Pronounced "IF-ah". Denotes a girl so hot you'd just fuck the shit out of her. Almost always directed at a slut you'd really have no respect for other than for the fact that she's got a bangin body and a pussy begging to get fucked to pieces.
MILF IDH: This is a really hot girl who is young but also happens to have borne children.
IDH MILF: Not to be confused with the former, but denotes a hot older woman who is a MILF. (Can be used intermittently with simply "MILF")
IRH: "I'd Rape Her" - Pronounced "EER-ha" A girl so hot that you'd just have to fuck her any means possible, but usually denotes a girl too young for consensual legal sex. Used always as a joke, but the girl is undeniably hot.
IDH/DIDH: Can be used jokingly when directed at a really fat or ugly person or male. You obviously would not ever touch the person, but it's funny to point out among your friends the ugly fat bitches at seaside or wherever.
F: Yo, I fucked that girl at the party last night!
J: The DIDH?
F: Nah... the IDH plus four B
J: Ohhh cool. Well so who is that now, August?
K: Wait, what?