Left speech radical (simplified from 言) - the indexing semantic. Heads verbs of speaking, agreeing, promising, judging: 说, 话, 谁, 谢. In 该 it points to the verbal sense of stating an obligation: words about what someone ought to do.
Right 亥 gives the sound - shares the rime with 该 gāi, with the initial shifting (h→g) but the rime preserved. A typical phonetic compound. Same phonetic body produces 孩 (child) and 骸 (skeleton); the 'pig / earthly branch' meaning of 亥 doesn't carry.
