How much is a knob of butter?
I was reading a question here on Food and Cooking and no one seemed to know how much it is. I tried to Google convert it to cups but no dice.
So how much is it?
In this Gordon Ramsey scrambled eggs video he uses a "knob" of butter. It appears to be about 2 Tbsp.
I don't think it's intended to be a specific term. You'd never see "knob" used for baking, where exact amounts matter. When cooking, recipes tend to be a general guideline rather than a strict set of instructions.
This page agrees, a couple tablespoons, but definitely not an exact measure: http://www.ochef.com/300.htm
In particular, it's a common English term. Jamie Oliver uses imprecise measurements all the time (a handful, a glass of wine, knob of butter). My (English) mother thinks this is in large part because the average Englishman thinks of a cooking as a woman's job or a bit queer for a man to be doing. To get around this, male chefs tend to be a bit nonchalant about things like measuring, leaving an "I'm a man, I just throw it together" kind of vibe.
As a chef who is queer as a three dollar bill, I don't think your mother is on the right track. (FWIW, my parents are English too). The nonchalance comes with confidence and knowing that a little more of this or a little less of that in home cooking is really going to make very little difference to the end result.
My mother cooked some things without measuring. My grandmothers often cooked without measuring. One advantage is that you can compensate for variations in ingredients and the weather, for example, if you're not locked into "precision" but more focused on achieving a desired result and have a sense of whether what you're doing is going to succeed. I'm "nonchalant" about it to some extent because there are a lot of things that don't require *any* precision and sometimes the result of a variation results in a pleasant surprise. Perhaps it's "Darwinian" cooking.
Looks more like he sliced a bit of butter off the stick. Precision is actually bad for these kinds of cooking, because the amount of butter also depends on things like the size of the pan, type of butter, or the other ingredients it's going with. Much like herbs and spices, you'll have to practice winging it.