An Etymological Dictionary of the German Language/Annotated/Block

by Friedrich Kluge

Block, masculine, ‘block, log, prison,’ from Middle High German bloch, ‘log, plant, a sort of trap.’ In the latter signification (to which Middle High German blocken, ‘to put in prison,’ is related) it represents Old High German bilóh (with syncopated i; see other similar examples under bei), ‘lock-up,’ which belongs to an Old Teutonic strong verb lûkan, ‘to lock’ (compare further English lock; see Loch). The meaning ‘log, plank’ (Middle High German bloch), is probably based on a different word, which is most likely related to Balken; even in Old High German, bloh occurs. The cognates passed into Romance (French bloc, bloquer), whence again Modern High German blockieren, English to block.