verb/ʁa.lɑ̃.tiʁ/
to slow down; to abate
“... Lothaire commença à ralentir peu à peu, par réflexion, ses allées et venues dans la maison de son ami.” — ... Lothario started to abate little by little, by reflection, his goings and comings to the house of his friend.
From a now obsolete alentir, corresponding to a- (“toward”) + lent (“slow”) + -ir (“verbal suffix”).