丫 has multiple uses: as a shape descriptor (Y-shaped fork), as part of 丫头 (girl, can be affectionate or derogatory), and in online/colloquial speech as a Beijing-flavored variant of the particle 啊.
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When used alone to mean 'girl,' 丫 can sound rough or dismissive. It originates from 丫鬟 (yāhuan), meaning a servant girl in old China.
The single vertical 丨 below is the indexing Kangxi radical, drawn as the trunk from which the two upper strokes diverge. It groups 丫 with other vertical-stroke entries and visually anchors the forking shape on its supporting stem.
The two splayed strokes on top picture a forked branch or pigtails parting in two directions. This visual fork is what gives the character its core meaning of 'Y-shape, branching, fork' and its colloquial use for braided pigtails on young girls.
