This idiom from the UK in the 70s means “Do your utmost!” In my youth, especially when my sister & I were sitting around, engrossed in a game of Cribbage or War [or some other frivilous activity], our father would ask us to fetch him a beer; and we would stall him, saying, “Aww, not now. When this game is over.” He would retort, “C’mon, how many Therbligs would it cost you?” The Therblig is a unit of effort, devised in 1908 by Frank Burke Gilbreth [inventor of Time & Motion analysis, and the author of Cheaper by the Dozen], to quantify units of work [such as search, find, select, grasp, hold, and so on]. It’s an anagram for “Gilbreth” (sort of). It’s definitely a metaphor.
Talk about “doing the math,” we all can make instant calculations of how many Therbligs it would cost us to do a task for a Loved One who asks us; and it is human nature to expect a quid pro quo. If we expend more Therbligs than we receive credit for [in praise, gratitude, success, financial recompense, or other forms of positive reinforcement], we experience the humiliation of having been “hoodwinked” [schmized] into doing something “for nothing.” Let’s say we use up a certain amount of Therbligs studying for a test, and then flunk it anyway. [Or training for an athletic event, and then lose.] For some individuals, the misery of having “put it all on the line” and still failed, is so mortifying that they write themselves a life-long, face-saving Note to Self: “Never Let Them See You Sweat.” In fact, the Note advises, “Make It Obvious That You Didn’t Try Very Hard, at All.” Then, if you don’t succeed, you have only the pain & suffering of your loss to cope with–not the mortification, as well.
It takes a certain amount of bravery, such as Lord Louis Montbatten urged, to risk the humiliation of defeat, by “giving it some welly”–going at your goal full-tilt boogie–knowing that the you may still be judged to have done the thing “badly.”