means In math, a space that looks flat and ordinary up close but can be globally curved or twisted; more loosely, something with many parts, forms, or facets (and in engines, the branched pipe that gathers or distributes gas to the cylinders).

from From Old English 'manigfeald' — literally 'many-fold,' built from 'manig' (many) plus '-feald,' the same fold-sense that gives us 'twofold' and 'threefold.' So the word is itself a picture of something folded many times over. The mathematical sense (German 'Mannigfaltigkeit,' Riemann's coinage in the 19th century) and the engine sense both grow from that same old image of layered, multiple folds.