A motherism is something one's mother said often or which is worth remembering (or hard to forget, even if you'd like to). One of my mother's motherisms was, "Be nice." Everything was nice or was supposed to be or whatever. I grew to really dislike that word because it seemed so shallow and artificial. Of course, it isn't completely those things, but I rarely use the word myself.
On the other hand, like many mothers (how about yours?), my mother had smart and useful things to say, too. Here's a little story about one of them.
Lately I’ve been applying that motherism to various aspects of my life. For example, I’m learning in my middle years that sometimes the best contribution I can make to a situation or to a piece of writing is to stop.