Can somebody explain the grammar behind this Colombian expression? Assuming it's the indirect object pronoun, to what or whom does the "le" refer?
And why is it missing a direct object pronoun? Why isn't it hágalo or hágaselo or hazlo?
Hágalo is do it by yourself
Hágaselo is do it to/for him/her
Hazlo is just do it.
Hágale is go ahead and do something.
-le refers to Usted
The pronoun can be omitted (hence is missing) since it would be too long to say hágale usted pues but in some cases the answer to ¡hágale pues! could be No. Hágale ustéd.
In Colombia this expression is used in the same way as upa usually to tell someone that is passive regarding some situation to go into action and do something.
Mother to child at 6:00 am: Mijo hágale pues levántese que lo coge la noche pa' ir al colegio. Son, hurry wake up or you are going to be late for school
Wife to husband: Hágale pues cambie el foco del baño que lleva quemado ocho días. Honey, get up and change the bathroom light bulb that's been broken for a week
Other use is on a dare
And lastly (and thanks to Rodrigo for reminding me) pues is an interjection that helps express the sense of urgency or need for an immediate action. i.e ¡Hágale! is less urgent than ¡Hágale pues!
After reading this Wikipedia entry for "órale," I am concluding that the "le" in "hágale pues" is in fact a grammatical expletive and not an indirect object pronoun.
Describing "órale" here's what the Wikipedia entry says:
The word’s origin is a shortening of “ahora”, meaning “now”, with the added suffix “-le”, which is a grammatical expletive – a word part that occupies a position without adding to the sense, i.e. “ándale” and “épale”
From a separate Wikipedia entry:
A syntactic expletive is a word that performs a syntactic role but contributes nothing to meaning