陪

Left 阝 is the left-side mound radical, the abbreviated form of 阜. It indexes terrain, settlements, and the act of being-alongside (the original sense was earthen hills people gathered against). Same family: 院 courtyard, 阵 formation, 险 steep, 阶 step. In 陪 it pictures the embankment one walks beside - physical accompaniment turned social.

Right 咅 supplies the sound - pǒu shifting to péi via well-attested labial drift. The same phonetic spawns 培 to cultivate, 倍 double, 部 section, 赔 to compensate, all sharing the péi/bèi/bù rhyme cluster. Pure phonetic loan; the original "spitting" sense of 咅 contributes nothing to "accompany".