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1.
To allow oneself to spend a lot of money
"You make me wanna take you out and let it rain" -Leavin' by Jesse McCartney
by MathewSi March 29, 2009
 
2.
Is exclaimed when a favourable event occurs, be it the success in an endevour or a humourous observation.
Fucking let it rain!!!!!! Choke on it YOU MOTHER!!!
by chris carter September 06, 2004
 
3.
A proclamation of confidence...that a conquest or 'win' is imminent regardless of the trails that may be faced along the way.

A provocation to opposing forces - 'do your worst...it doesn't matter...I'll reign supreme anyway'.

'Bring it'.
(A Golf Game- at tee off)

Robert: I'm all over this golf caper. Are you ready for the right caning thats coming your way James?

James: Let it rain Robert...let it rain...
by Chuck McGee August 12, 2009
 
4.
Used to exclaim any matter of emotions, examples of use mostly evolve from the film 'Along came Polly', following the scene where Sandy Lyle shoots some hoops and verbally expresses all of his conclusive emotions in a variety of phrases including "Let it rain", "rain dance" and "white chocolate". The phrase is also often used by females from Spalding High School in many situations, and has a high comedy value.

A hand movement sometimes accompanies the phrase; this could be described as a mid-air spanking.
"let it rain, RAIN DANCE!!"
by rochelle90 August 19, 2008
 
5.
when a person absolutely rips someone at a sport (usually being basketball !) and score a "basket/point/goal/try/touchdown"
kevin rips the whole class at basketball in P.E. and in the middle of a lay up yells out "Let It Rain!"
by random!!! February 19, 2010
 
6.
Let it rain is from the film along came polly when they are playing B-
ball meaning good luck
Let it rain "rain dance"
by Patoola the grey December 22, 2004