let 1 
(lĕt)
(lĕt)v. let, let·ting, lets
v.tr.
1. To give permission or opportunity to; allow: I let them borrow the car. The inheritance let us finally buy a house. See Usage Note at leave1.
2. To cause to; make: Let the news be known.
3.
a. Used as an auxiliary in the imperative to express a command, request, or proposal: Let's finish the job! Let x equal y.
b. Used as an auxiliary in the imperative to express a warning or threat: Just let her try!
4. To permit to enter, proceed, or depart: let the dog in.
5. To release from or as if from confinement: let the air out of the balloon; let out a yelp.
6. To rent or lease: let rooms.
7. To award, especially after bids have been submitted: let the construction job to a new firm.
v.intr.
Phrasal Verbs: 1. To become rented or leased.
2. To be or become assigned, as to a contractor.
let down
1. To cause to come down gradually; lower: let down the sails.
2. To withdraw support from; forsake.
3. To fail to meet the expectations of; disappoint.
let on
1. To allow to be known; admit: Don't let on that you know me.
2. To pretend.
let out
1. To come to a close; end: School let out early. The play let out at 10:30.
2. To make known; reveal: Who let that story out?
3. To increase the size of (a garment, for example): let out a coat.
let up
Idioms: 1. To slow down; diminish: didn't let up in their efforts.
2. To become less severe or intense: The rain let up.
let alone
Not to mention; much less: "Their ancestors had been dirt poor and never saw royalty, let alone hung around with them" (Garrison Keillor).
let go
To cease to employ; dismiss: had to let 20 workers go.
let off on Informal
To cause to diminish, as in pressure; ease up on: Let off on the gas so that we do not exceed the speed limit.
let (one's) hair down
To drop one's reserve or inhibitions.
let (someone) have it Informal
1. To beat, strike, or shoot at someone.
2. To scold or punish.
let (someone) in on
1. To reveal (a secret) to someone: They finally let me in on their plans.
2. To allow someone to participate in (something).
let up on
To be or become more lenient with: Why don't you let up on the poor child?
[Middle English leten, from Old English lǣtan; see lē- in the Appendix of Indo-European roots.]