shape
From TidalCycles userbase
Type: shape :: Pattern Double -> ControlPattern
shape produces wave shaping distortion, a pattern of numbers from 0 for no distortion up to 1 for loads of distortion. It can get very loud if you reach 1 - be careful!
d1 $ sound "feel*4" # shape "0 0.2 0.4"
d1 $ sound "bd*16" # shape (range 0 0.5 $ saw)