This custom can be traced back to the Tang dynasty.
Tips
usage
The literal sense is moving against a current; the everyday sense is figurative, mainly in 追溯 (to trace back) and 回溯 (to look back over). Think of mentally swimming upstream through time.
Left water radical, the side-form of 水. The base image is moving against a river current, so the radical fixes the watery origin even when the modern use is figurative tracing.
Right 朔 supplies the sound — shuò drifting to sù. It can carry a faint 'turning back to the beginning' flavor, matching the upstream sense; the same phonetic appears in 塑.
