delusively+gradual
1insidious — a. 1. Artful, sly, wily, guileful, arch, crafty, cunning, subtle, intriguing, designing, deceitful, treacherous, crooked, trickish, tricky, foxy, snaky, diplomatic, Machiavelian. 2. Secret, deceptive, that creeps upon one unawares, delusively… …
