From sneaking + -ly.
sneakingly (comparative more sneakingly, superlative most sneakingly)
- In the manner of one who is sneaking or sneaky; slyly, covertly.
1930, Norman Lindsay, Redheap, Sydney, N.S.W.: Ure Smith, published 1965, →OCLC, page 82:"Do they not sneakingly bestow on me their crass inability to do anything with their own misbegotten progeny, a subterfuge which I scornfully fub off on text-books?"