Mid 19th century. From out- + lash, afterto lash out.
outlash2
/ˈaʊtlaʃ/
verb
[no object]To lash out; to strike out violently. In earliest use figurative : to break out into excess; to be extravagant, exaggerate. Compare overlash. Now rare.
Origin
Late Middle English (in an earlier sense). From out- + lash.