British English* expression.
This little bit of English idiom means that you are barely surviving, or subsisting, living hand to mouth. Doing something "on a shoestring" suggests that very little money is available in the budget, and that one is getting by with whatever means available. The expression may have originated in gambling circles, "shoestring gambler" being used in the late 19th century. The Encyclopedia of Word and Phrase Origins states that one's resources could be limited to shoelaces. Someone had the shirt off your back, maybe?
To do anything on a shoestring means to tighten the grip on cash and achieve the end as frugally as possible. It's a common state of being for many, especially students, who frequently complain of their impecunious state, hence the cheap booze in student bars. <mutter>Mind you, some of them don't know how to tie shoelaces</mutter>.
*TheBooBooKitty advises me that it is current usage in the US, for which I thank him.