As a Noun:
1. a device for securing a door, gate, lid, drawer, or the like in position when closed, consisting of a bolt or system of bolts propelled and withdrawn by a mechanism operated by a key, dial, etc.
2. a contrivance for fastening or securing something.
3. the mechanism that explodes the charge; gunlock; safety (def 4).
4. any device or part for stopping temporarily the motion of a mechanism.
5. an enclosed chamber in a canal, dam, etc., with gates at each end, for raising or lowering vessels from one level to another by admitting or releasing water.
6. an air lock or decompression chamber.
7. complete and unchallenged control; an unbreakable hold.
8. Slang. someone or something certain of success; sure thing: He’s a lock to win the championship.
9.Wrestling. any of various holds, especially a hold secured on the arm,leg, or head:
10. Horology. (in an escapement) the overlap between a tooth of an escape wheel and the surface of the pallet locking it.
11. Metalworking. a projection or recession in the mating face of a forging die.
As a Verb (used with an object):
12.to fasten or secure (a door, window, building, etc.) by the operation of a lock or locks.
13.to shut in a place fastened by a lock or locks, as for security or restraint.
14.to make fast or immovable by or as if by a lock: He locked the steering wheel on his car.
15.to make fast or immovable, as by engaging parts: to lock the wheels of a wagon.
16.to join or unite firmly by interlinking or intertwining: to lock arms.
17.to hold fast in an embrace:She was locked in his arms.
18.to move (a ship) by means of a lock or locks, as in a canal (often followed by through, in, out, down, or up).
19.to furnish with locks, as a canal.
As a Verb (used without object)
20. to become locked: This door locks with a key.
21.to become fastened, fixed, or interlocked : gears that lock into place.
22. to go or pass by means of a lock or locks, as a vessel.
23. to construct locks in waterways.
So…of course I couldn’t write just ONE haiku. I give you two extremes of this very complex four-letter word.