Ach is an interjection expressing grief, distress, or lamentation — equivalent to "Ah!" or "Alas!" in English. It occurs in prophetic laments and calls to repentance, giving voice to deep sorrow. Not to be confused with the more common H251 (brother/ach).
Lament is a legitimate and honored form of prayer and expression in Scripture. The prophets cried "Ah, Lord!" (Jeremiah 1:6; 14:13) not as blasphemy but as raw, honest petition before God. This interjection models the biblical permission — even the imperative — to bring our grief and dismay directly to God rather than suppressing it.