Foolishness is nonsense or stupidity; it's acting in a way that is silly or irrational. Throwing a temper tantrum because the lunch line is moving slowly is pure foolishness — your outburst won't make the line move any faster.
When your actions show a lack of good sense or solid judgement, you're betraying your foolishness. Impulsively jumping off the garage roof into a pile of leaves is evidence of your foolishness. We can trace this word's origin back to fool and the Old French fol, "idiot," and also "blacksmith's bellows," from a root meaning "blow or swell."