kʷer-s-eyo-
“to keep running, racing”course, current, excursion, recur
Iterative of *kʷer-s- giving Latin currere, English course, current, excursion, recur, occur.
Discussion
The PIE iterative form *kʷer-s-eyo- (to keep running, to race repeatedly) derives from *kʷers- (to run), with the *-eyo- suffix marking habitual action. The meaning is "to be in the continuous state of running" — running as an ongoing activity rather than a single sprint.
Latin currere (to run) continues the root and gave English one of its most productive derivative families: current (that which runs — water, electricity, time), currency (money that runs/circulates), course (a running — a racecourse, a course of study, a course of a meal), occur (ob-currere, to run toward — to happen, as events "run toward" you), recur (to run again), concur (to run together — to agree), incur (to run into — to bring upon oneself), discourse (dis-currere, to run apart — then to speak at length), excursion (a running-out — a trip), precursor (one who runs before — a forerunner), and corridor (a running-place — a passage).
The English word car (from Latin carrus, possibly from a Celtic form of the running root) and career (originally a racecourse, then a life's professional course) may also connect.
English horse (OE hors) is NOT from this root (its PIE origin is debated), but the concept of running overlaps with the PIE horse vocabulary through the equestrian culture that valued speed.
The root connects to *drem- (to run — the drome/hippodrome root) and *peth₂- (to fly/rush) as part of the PIE vocabulary of rapid movement.