Bottom 儿 is the human-legs radical and the entry's indexing radical. Together with the foot above, it pictures a person whose foot has stepped out in front of the body, giving the core meaning go first, ahead, earlier.
semantic
先xiān
footprint-on-person (upper fused body)
The top four strokes are a fused contraction of 之 (a footstep) sitting on the body. Historically 先 showed a foot stepping ahead of a person below; the upper portion is now a single indivisible silhouette of that footprint shape.
