The postfix
-al in English means
"of or pertaining to". It comes from Middle English, from either Latin or French, possibly from Etruscan before that. It's often added to nouns to make an adjective form. The postfix
-ism (from Ancient Greek) is a suffix added to the end of a word to indicate that the word represents a specific practice, system, or philosophy, often political ideologies or artistic movements. It suggests the word is related to a belief accepted as an authority by a group or school of thought.
The word brutal comes from Medieval Latin brutalis ("savage, stupid"), from Latin brūtus ("dull, stupid"). The English use of the word has a particular connotation/definition that differs from both the French and original Latin.
The French word brut comes from Middle French, from Old French, from the Latin brūtus (“heavy, dull”). Today it means either "raw", "strong", or "gross" ("big, thick, large, stout", "whole, entire, overall, total, aggregate").
The word brutalism could translate as "a system, belief, or artistic movement, of or relating to, a whole/entire, dull/big/thick stupid/savage aggregate". Doesn't make for a great definition.
However, that's not the whole story.
The word brutalism was coined first by Swedish architect Hans Asplund in 1950 as nybrutalism. It described a home Villa Göth (built by Bengt Edman and Lennart Holm) made using red brick - not concrete. In 1955, British journalist Reyner Banham popularized an essay called “The New Brutalism,” in which he used the brick-and-steel Hunstanton School (by architects Peter and Alison Smithson) as an example of this new form. The point was to stretch Modernism even further, but concrete was not (yet) involved.
In 1966, in French architectural critic Reyner Banham's book "The New Brutalism: Ethic or Aesthetic?", he redefined the word by suggesting that brut was a French pun, combining the meanings of "béton brut" and "brutal". 16 years after the coining of nybrutalism, the term's use became linked to concrete.
So really, the word originated as a reference to the the raw edges of an extended Modernist design, and then was reinterpreted as a pun relating to raw concrete, and then the English who weren't aware of this pun simply assumed it related to the brutality of the buildings' aesthetics.
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ps. brutism would (without a different explicit definition, use, or origin) be the same word as brutaism, just without the "of or pertaining to" postfix. English has thousands of words which have different spellings and the same meaning. You can replace the -al postfix with -ition or -ision and it means the same thing (disposition/disposal=dispose, proposition/proposal=propose, submission/submittal=submit, transmission/transmittal=transmit). But not in all cases, as some forms end up having a different origin (reversion/reversal=revert,reverse).