Left-side 阝 is the mound radical 阜 in compressed form — always meaning hill or earthwork on the LEFT (right-side 阝 is the different city radical 邑). Indexes 阿 with 陡, 阴, 阳. Original sense was a sheltered curve under a hill.
Right is 可 supplying the sound — kě drifting to ā through k/zero-initial alternation. Same phonetic family as 河, 何, 哥. The familiar-prefix sense detached from the original hill-nook meaning.
