swell. vb, swells, swelling, swelled, swollen or swelled. 1. to grow or cause to grow in size, esp as a result of internal pressure. Compare contract 1, contract 3. 2. to expand or cause to expand at a particular point or above the surrounding level; protrude. 3. to grow or cause to grow in size, amount, intensity, or degree: the party is swelling with new recruits.