verb/la.ʃe la pʁwa puʁ l‿ɔ̃bʁ/
to give up what one has already got to go chasing rainbows; to lose everything because one wanted too much; to get sidetracked, to lose sight of what is important
Literally, “to let go of the prey for the shadow”; from the fable Le Chien qui lâche sa proie pour l'ombre by Jean de La Fontaine.