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1.
Chuckling silently to myself. Used similarly to lol, but when you're not actually laughing out loud.
Yeah I was cstm when I heard about that.

Cstm Bullshaz, you n00b.
by [0SC]Just John May 03, 2007
 
2.
Chuckling Softly To My Self. Generally used as a response to a humerous situation or communication via e-mail or instant messaging. Used especially in the office setting where LOL (Laughing out loud) would rarely be the actual or appropriate response.
A co-worker hears another co worker say something innocently funny like "do do" (doo doo) or "to to" (tutu) on the phone with a customer and shoots you a message saying "He just said doo doo." The appropriate response and reply could be "CSTMS"
by Virgil Brazle August 20, 2008
 
3.
Chuckle Silently To Myself

"Lol" is an often misused term. Many people write "lol" when they don't actually laugh out loud. In reality, the term "cstm" is a more accurate description of the virtually silent noise that escapes one's airway when viewing or hearing something of a semi-hilarious nature. This aim of this term is to make society more honest about the nature of its reactions.
Wow, your facebook status is so funny! It totally made me cstm!!
by therealburninator March 28, 2009
 
4.
Chuckling Softly To Myself
It wasn't so funny that I was LMAO, but funny enough to be CSTM
by badboydano September 21, 2011