- (UK, Australia) Sorry mate, I didn′t see you.
The acronym is used by motorcyclists and cyclists to describe traffic collisions in which the driver of a car or larger vehicle claims not to have noticed a two-wheel biker. Usually derogatory, in that the intended implication is often that the driver was not at fault (when he/she patently was). SMIDSY was also used as a title of a BBC series on road safety broadcast in the 1980s.
A related usage by the UK government is: LBFTS, for 'looked but failed to see'.