(1) Unexciting, unadventurous, mainstream or dull. Generally used in describing a person with such attributes or who plays it safe. Historically, derived from a person whose goal in life is to have three "squares" a day (see definition (3) below), a roof over his head, a respectable job, a house, 2.5 kids, etc. Became a hip/popular expression of derision of the mainstream in the beat generation.
(2) Proper, decent, straightforward, acceptable, even.
(3) A regular and decent meal.
(1) Naw, he's a square; he won't dance (do anything exciting), he'll just shoot crows (go home).
(2) Hey man, thanks for takin' care of that, you're all square with me now.
(3) Dack is joining the army; at least he gets a dry bunk and three squares a day.
(2) originally, short for gentleman or gent.
(1) What up G?
(2) What up G?
(It works the same for both.)