Lindsay Lohan -- Sexed Up for Halloween Party at Playboy Mansion

Lindsay Lohan's been a very naughty girl ... and this weekend, she dressed the part ... to kick it at the annual Halloween party at the Playboy Mansion.

Seemingly inspired by her cleaning duties at the morgue, Lohan donned her sexiest French maid uniform Saturday night ... and hung out with a bunch of other ladies who've bared all in the mag.

TMZ broke the story ... Playboy shelled out nearly a million bucks to get Lindsay to strip down to her birthday suit. Still no word on when the spread is expected to hit the shelves.