Green’s Dictionary of Slang

(US black) neg;lected, overlooked.

2001		Source Aug. 32: The very slept-on Queen Pen, whose bio would make one helluva movie.
2007		B. Coleman Check the Technique 344: ‘[T[he remix of that song [on the single, the ‘tryandblendit Remix’] is one of the most slept-on things that PRT ever did’ .