Kukkucca: lit. 'wrongly-performed-ness' ku+krta+ya i.e. regrets, remorse, uneasiness of conscience, worry, is one of the kammically disadvantageous akusala mental abilities Tab. II. which, whenever it arises, is associated with hateful discontented consciousness Tab. I and III, 30, 31. It is the 'repentance over wrong things done, and right things neglected' Com. to A. I. Restlessness and regrets uddhacca-kukkucca combined, are counted as one of the 5 mental hindrances nivarana, (Five hindrances).
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.