1869, to denote the shape of the letter V. As a type of engine, by 1915.
1877, French form of ton (n.1), adopted for English use to denote a metric ton (1,000 kg.).
"tough outer membrane surrounding the brain and the spinal cord," c. 1400, from Medieval Latin dura mater cerebri, literally "hard mother of the brain," a loan-translation of Arabic umm al-dimagh as-safiqa, literally "thick mother of the brain." "In Arabic, the words 'father,' 'mother,' and 'son' are often used to denote relationships between things" [Klein].
1670s, "filled space, the fullness of matter in space" (opposite of vacuum), from Latin plenum (spatium) "full (space)," neuter of adjective plenus "full, filled, greatly crowded; stout, pregnant; abundant, abounding; complete," from PIE root *pele- (1) "to fill." Used to denote fullness in general, hence the meaning "of a full assembly of legislators" is recorded by 1772.
1924, in Joan Riviere's translation of Freud's "Das Ich und das Es" (1923), from Latin id "it" (as a translation of German es "it" in Freud's title), used in psychoanalytical theory to denote the unconscious instinctual force. Latin id is from PIE pronominal stem *i- (see yon).