Sidney "Sid" Garner is the deceased father of Tracy Garner and Alan Garner. He was portrayed by Jeffrey Tambor. He is a supporting character in the original film, and a minor character in the subsequent films, appearing briefly in each one.
Sid was a well-liked and respected person. Unlike your average father-in-law, Sid was understanding, laid back, and hip.
Sid spoiled his son by letting him stay home and pay Alan's rent up to where Alan stayed unemployed even at age 42 out of laziness. The nickname Sid gives to Alan is "Sweetie". Sid lends his prized Mercedes convertible to Doug for the Las Vegas trip and he told Doug to only let him drive it because he doesn't like Phil and he knows that there is something wrong with Alan, however it is later revealed that the car was his wedding gift to Doug and Tracy. Sid knows all about Las Vegas, he knows once you leave, you come out with something and especially gambling.
In the Hangover Part III, Alan has bought a giraffe but it is beheaded when he tows it under a low bridge at Irvine, California and closes the interstate. This forces Sid to write a large number of huge checks for repairs, which apparently has significantly reduced his wealth. Sid, angry at Alan for not owning up to his mistake, orders him to take his ADHD medication again (which Alan quit). Alan refuses, which finally causes Sid to snap and in a fit of rage, Sid collapses and dies of a heart attack, much to the horror of his wife Linda and their housekeeper. Alan, however, was present during the tragedy but was not aware of what was happening as he had put on his headphones to ignore Sid's lecturing. A funeral was held for him with his wife crying and Alan giving a speech upon missing him.
Initially, Alan failed to understand that his immature behavior and mentality played a vital role in his father's death but as he comes to terms with the fact that Sid is gone, he realizes that he let him down and began to re-evaluate his life and changed for the better.
"Remember what happens in Vegas stays in Vegas. Except for herpes. That shit'll come back with you."
"They're probably on a heater. And you never walk away when you're on a heater."