Top bamboo radical - squashed-down form of
竹 used when bamboo sits on top. The indexing radical of
签 (Kangxi #118). Carries the literal etymology: the original
簽 was a bamboo strip used as a tally, label, or sign-in slip. Same radical anchors
笔 (brush),
笑 (laugh),
答 (reply),
简 (bamboo strip, letter) - the entire bamboo-implement family.