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The lock that the keyed shaped as a F, opens.
I can think of two kinds of LOCKS. Take your choice ~
1) A door lock which is in Latin, sera (f). ... To lock a door use the transitive verb, obserare.
2) A lock of hair is coma (f).
The F Lock on a keyboard disables the use of function keys. E.G: F1 F2 F3 etc.
There are Spinebusters F-Us F5s Leg lock and much more
Click F5 and make sure you have F lock on if you have it.