The modal verb 'can' has changed a bit over the history of English.

We see from its Old English ancestor, cunnan, that its meaning was once 'to know' rather than 'to be able'.

This earlier meaning can still be found in related adjectives like 'canny', 'cunning' and 'uncouth'.

Thanks to all this, we can make etymological connections to similar words that also convey an idea of knowledge.

For example, there's 'know' itself and 'ken' in English, while elsewhere in the Germanic family we have words like German kennen, Yiddish kenen and Swedish känna.

However, to find cognates outside Germanic, the first step is to undo Grimm's law and change the /k/ sound to /g/.

Et voilà - there's Greek gignṓskō, from which we get words like 'diagnosis' and 'agnostic', and also Latin gnōscō, from which come 'ignorant' and 'recognise'.