Someone who spends a lot of time in bed; this is not because they’re ill, but because they’re a bone idle, useless, twat who is totally unfit for purpose as part of the human race.
“Malcolm is always late for work; he says he’s got chronic fatigue syndrome.”
“Nah, he’s just a uselessslug-a-bed cunt.”
A threesome as a one-off (so to speak). A three-in-a-bed is a sexual encounter between three people, usually (at least for two of those involved) for the first and last time.
A phrase beloved of the British tabloid press is "three-in-a-bed romp", which stresses bother the impermanence and the (just-for-)fun nature of the experience, thus engaging the sexual interest and desire, the envy and thus the moralising indignation of the newspaper's readers all at once!
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.