TO LOCK - [verb] to fasten something, usually with a key; to put something in a place and close it with a key
Examples of use
He had never locked the door for a night before there was a burglary in his flat.
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English speaking practise
Remember that if you don't use that word in English conversations or writing you'll forget it in a short time. So the best thing you can do to learn this word is just trying to use it in speaking or writing. Remember also that practise is the key to fluent english speaking.
You can always find a partner, who would also like to improve his/her