Ephraim, the largest of the ten tribes, is put by Figure of speech Synecdoche (of the Part), App-6, for the whole.
whoredom. idolatry. See note on Hosea 1:2.
Related Commentaries of Hosea 5:3
Hosea 5:3
3 I know Ephraim, and Israel is not hid from me: for now, O Ephraim, thou committest whoredom, and Israel is defiled.