Etymology
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latch (n.)

"device for catching and retaining," especially "a fastening for a door," late 13c., probably from latch (v.).

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latch (v.)

Old English læccan "to grasp or seize, catch hold of," also "comprehend," from Proto-Germanic *lakkijanan. Not found in other Germanic languages; according to Watkins probably from PIE *(s)lagw- "to seize" (see lemma). In its original sense the verb was paralleled and then replaced by French import catch (v.). Meaning "to fasten with a latch" is mid-15c. Related: Latched; latching.

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-on 

subatomic particle suffix, from ion.

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on- 

the preposition and adverb on used as a prefix; Old English on-, an-.

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on (prep., adv.)

"in a position above and in contact with; in such a position as to be supported by;" also noting the goal to which some action is or has been directed; "about, concerning, regarding; in a position to cover;" as an adverb, "in or into a position in contact with and supported by the top or upper part of something; in or into place; in place for use or action; into movement or action; in operation," Old English on, unstressed variant of an "in, on, into," from Proto-Germanic *ana "on" (source also of Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- (1) "on" (source also of Avestan ana "on," Greek ana "on, upon," Latin an-, Old Church Slavonic na, Lithuanian nuo "down from").

Also used in Old English in many places where we now would use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877.

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clip-on (adj.)

"held on by means of a clip," 1909, from the verbal phrase; see clip (v.2) + on (adv.).

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hang on (v.)

1860, "to remain clinging," 1860, especially "cling fondly to" (1871); see hang (v.) + on (adv.). As a command to be patient, wait a minute, from 1936, originally in telephone conversations.

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latch-key (n.)

also latchkey, "a key to raise or draw back the latch of a door" and allow one to enter from outside, 1825, from latch (n.) + key (n.1). Latchkey child first recorded 1944, American English, in reference to children coming home from school while both parents are away at work.

Many elementary school principals and teachers have always known the "latchkey" child or the "eight-hour orphan." [New York State Teachers Association, "New York State Education," 1944]

The older or simpler device was a latch-string, which could be pulled in to lock up; having it out was symbolic of openness.

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hard-on (n.)

"penile erection," 1922, earlier as an adjective (1893), from hard + on.

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