to Thrash is pretty much to violently beat, flog or plunge/flail around. (imagine a kid rolling on the ground and hitting things crying because he didn't get what he wanted for his birthday) but since Florence and the Machine are a UK band they may be using it as a slang term(which means party...usually with drinks involved) it seems to me though the lyrics is saying the problem with sleeping pills. Don't touch the sleeping pills, they mess with my head Dredging of big white sharks, swimming in the bed And here comes a killer whale, send me to sleep Thrashing the covers off, has me by its teeth thrashing here has 2 meanings(from what I understand from the song), 1. the act of violently taking off the cover to the sleeping pills(by the singer) 2. unveiling the violent pain the singer is enduring while unable to stop using the sleeping pills nice song by the way, and oh, a link to Wiktionary which is pretty awesome http://dictionary.reference.com/browse/thrash