Asymmetrisk – ‘asymmetrical’ must be the word most difficult to spell in the Swedish language. Even on cultural pages in otherwise trustworthy newspapers it is more often than not spelled assymmetrisk. When I find it I immediately lose confidence in the writer and stop reading the article.
Asymmetrisk comes from Greek. The prefix a- is a negative, 'non-'. The Swedish relative is o-, the English and German is un-, Sanskrit a-, Latin in- .You could say osymmetrisk in Swedish or unsymmetrical in English (or maybe even insymmetrical) and immediately understand what it means even if those words are not in the dictionary.
So where does this unnecessary ‘s’ come from? The Latin preposition ad- means ‘to’ or ‘at’. When it is used as a prefix it is often assimilated with the subsequent consonant, e.g. affectus - ‘affect’, aggressio - ‘aggression’, alludere - ‘to allude’, approbare – ‘to approve’, attentio’ – attention. There are many such combinations with words starting with s-, e.g. assensio – ‘assent’, assiduus – ‘assiduous’, assigno – ‘I assign’, assimulo – ‘I assimilate’ assumo – ‘I assume’ etc. So for some strange unreason this double s has trampled into a field where it has no right to be.
(The Latin equivalent of Greek a- is in-, so there can be some confusion here too, as Latin in- can mean both non- and in, related to English in, and Swedish in and i.)