The phrase "no strings attached" refers to the act of doing something without asking for anything in return.
Example of use: "Danny said he'd do your chores today, no strings attached!"
The origin of the idiom “no strings attached” is directly tied to 18th century fabric merchants who would mark flaws in woven cloth by tying small strings to the bottom of the bolts at the locations where flaws were present. When a tailor or dressmaker needed flawless cloth, he or she would ask the merchant to provide them with fabric “with no strings attached.” This custom persists today.