The title of this post is an old saw about heralds, and a play on the words "can't" (or cannot) and "cant" (meaning "a rebus,* or pictorial pun").
What brought this post about was an article posted last August on "The Heraldic Rebus, Born in Battle and Embraced by Tudor England, Lives on in Your Smartphone", and subtitled "Expressing your identity in a clever string of images was a thing long before emojis".
The article, by Blair Mastbaum and published by Atlas Obscura, goes into the long history of rebuses, before noting their use in heraldry and other places, and ends up by talking about some modern versions that today's youth are using emojis, as earlier generations might have used rebuses, to refer to themselves or their friends in text messaging. For example, Isla Burgess is referred to by her friends by using the emojis for island and then iceberg in texts.
Anyway, the article is an interesting view of the history of such rebuses or cants, and well worth the read. You can find it on-line at https://www.atlasobscura.com/articles/heraldic-rebus-puzzle
* And just what is a "rebus", you ask? Well, it comes from the Latin phrase non verbis sed rebus, which means “not by words, but with things.” A rebus is a visual puzzle in which a word, or part of a word, is represented by a picture. When done in heraldry, on a coat of arms, this is called armes parlantes ("talking arms", where the charges on the shield "speak" the name of the bearer), or canting arms.
My favorite historical American canting arms are those of colonial merchant and well-known signer of the Declaration of Independence John Hancock, whose arms bear a hand and on a chief three roosters, or cocks, thus completing the pun on the surname, hand + cock. (The image below is from his memorial in Granary Burying Ground, Boston, Massachusetts, with his coat of arms at the top.)