Undergoer is from undergo + -er. Undergo is from Old English undergān ('to undergo, undermine'), from under- + gān ('to go'). Important recorded forms include -er (Old English), under (Old English), and gān (Old English). Morphologically, the under- element contributes the sense of beneath, lower rank, concealment, or insufficiency, depending on the base it modifies. The modern sense is largely compositional, with the historical elements narrowing into a stable English lexical item. In current use it covers one who undergoes or experiences something, especially an action. The wider family includes related words such as undergo and undergone.