Is 4-1 a thrashing? I don't think it is. 3-0 wouldn't be deemed a thrashing. 3-0 is, well beaten.
I mean, obviously you have to take into account the manner of defeat but if one teams beats another by, say, 2 goals to 0 (but out played them for 90 minutes), it still wouldn't be described as, a thrashing.
Personally, I think 4+ without reply is a thrashing.
Having said that, did we 'thrash' PSG?
I'd say we 'battered' PSG.
Cheers.