Variant or alternative spelling of 'caustic,' meaning capable of burning or eating away at material, or sarcastically cutting in words.
From Latin causticus, from Greek kaustikos (able to burn), derived from kaiein (to burn). The -al suffix marks it as an archaic or variant adjectival form used in older English texts.
In older scientific writing, you'd see 'caustical' instead of the now-standard 'caustic'—this shows how English streamlined its adjectives, dropping variant forms to make the language cleaner and faster to write.
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