steam [OE] In Old English steam was any kind of hot vapour or gas, and did not settle into the modern meaning until the 15th century. The phrase let off steam, meaning ‘to get rid of pent-up energy or strong emotion’, originated in the context of steam engines in the early 19th century. The literal meaning is ‘to release excess steam from a steam engine through a valve’, vital in preventing the engine from blowing up. The meaning which is familiar today arose in the 1830s in the alternative version blow off steam. Other phrases that recall the days of steam engines include get up (or pick up) steam, run out of steam, and under your own steam.