lessen /ˈlɛsən/verb 2conjunction 1forms 5 verb2 •To make less; to diminish; to reduce.December 6, 1709, Francis Atterbury, a sermon preach'd before the sons of the clergy at their anniversary-meeting in the Church of St. Paul St. Paul chose to magnify his office when ill men conspired to lessen it. •To become less. conjunction1 •(dialect, nonstandard) unless.She was fine-looking; he couldn't find a fault with her 'lessen he made it up. — 2011 formslessened · lessening · lesseneth · lessens · lessenest