In English, we have prefixes, like "pre-"; suffixes, like "-ize"; and arguably, expletives that function as infixes (one classic example is "abso-fucking-lutely"). In Japanese, we also have ...
Many verbs come in pairs, frequently but not always transitive/intransitive pairs. These verbs generally have multiple okurigana characters, but according to my dictionary one of the pair was formerly ...
On chat, Chocolate helped me find some examples of adjectives produced from verbs using the しい suffix. In the following examples, it appears to attach directly to the 未然形: 勇む → 勇ま ＋ しい 悩む ...