I believe it has fallen out of popular usage, kind of like “the bee’s knees” (and perhaps of similar vintage?).
I usually run across it in old movies.
I use pill, card, etc. Someone pointed out to me recently that I use “cross,” which struck him as really old-fashioned, though it just sounds normal to me.
I always assumed “pill” was from “a bitter pill to swallow.”
My mum and her Scottish cronies used ‘Pill’ or ‘Pillock’ interchangeably. I tend to say ‘Pillock’ more.
Same here. My mother used to call my sister and me pills all the time. Implication: pain in the ass. I never used the phrase as an adult until I started working with small children. It’s code between the parents and me for the kid being grumpy, misbehaving, or prone to tantrumming.
Same here. I’m 62 and use it, but rarely and only with talking to people of my age.
I use it. I think old phrases are the cat’s meow.
Glad I could be of assistance. 
It really does describe that certain kind of pain-in-the-ass-ness, doesn’t it? Power struggly, “not hearing” when told not to do something, etc.
I’m sure we’ll continue using it after he turns 3 (which will be a while yet), because I seriously doubt that the behavior will magically stop with the birthday…
Another vote for Mom. I always inferred a person who was being difficult for no good reason.
We used to use it a lot: “A pill is a pill and it always will.”
It’s been supplanted by “web-blanket”