To say something boldly or decisively — to declare or assert strongly without hesitation.
彼は『やってみせる』と大声で言ってのけた。
He loudly declared, 'I'll show you I can do it.'
そんな危ないことはしないと言ってのけたが、本心はわからない。
He asserted that he wouldn't do anything dangerous, but you can't be sure of his true feelings.
面接で彼女は自分の強みをはっきり言ってのけた。
In the interview she clearly stated her strengths.