Adjective (negative). Used to describe any activity that involves spending too much money, drinking too much, doing too many drugs, hanging out with dregs of society people, and generally any activity that involves lowering one's standard and hindering one's progress.
"A
greasy night out" - a night out that involves enormous expense and negative health consequences but very little pleasure as
compensation. Typically occuring in London (because it's so expensive and overcrowded) a
greasy night out would involve leaving the office at 6, downing 10 pints of Stella, spending fortunes on cabs to go to crap clubs where you queue outside for hours, then buy more overpriced drinks inside, stagger outside, may or may not visit a kebab house or a whore house, then get more expensive cabs home, and wake up in the morning to find you've lost you brand new Armani overcoat.
Can also be applied to other situations, such as greasy mates (one's friends who encourage the above, but have no means to pay for it so you must pay for them aswell. They act as a kind of grease
multiplier).