Are there any rules/patterns for selecting "ish" vs "like" as a suffix?

This question arose on a comment thread over on ELL. Are there any rules or patterns for when we should/do select "ish" as a suffix, rather than "like"? Some examples discussed were that birdlike comes naturally and is easily understood to mean like or in a manner such as that of a bird, but no native speaker would come up with birdish. As for the opposite, there are words such as cliquish, which clique-like would never be substituted for.

Another interesting point that FumbleFingers raised in that discussion is that we have some words which take the suffix ish but clearly and recognizably do not mean "x-like", such as "sheepish". (Perhaps the origin of sheepish is something along the lines of that sheep (the animal) act sheepish, but when you say "He looked away sheepishly" you don't mean "he looked away in a manner similar to that of a sheep.")

Solution 1:

In terms of etymology, -ish is a diminutive. It often carries a negative context: childish, churlish, clownish, cliquish, selfish, foolish, boyish, girlish, sheepish.

Compare with: adultish, heroish, geniusish ... these don't work. Adult-like, hero-like, genius-like: these sound more correct.

Compare directly childish and child-like. The former has a negative connotation (acting spoilt). The latter has a more neutral or even positive connotation (physical appearance, innocence, purity).

So though it's not a hard-and-fast rule, -ish, with its roots as a diminutive, is often used as an adjective that expresses negative traits of the noun to which it is appended. On the other hand, -like (and -esque) refer to a more neutral, objective resemblance to the noun.

Another use of -ish would include approximations for colours – such as greenish, bluish, redish – and quantities – such as fortyish, fiftyish.

A related use of -ish is to indicate "almost", particularly when paired with an adjective: cleverish, tallish, smallish, darkish, etc. (This is slightly different from approximation in that fortyish would indicate "around forty, possibly over", whereas tallish would indicate "almost or barely tall".)

All cases relate to -ish being diminutive-ish.

Solution 2:

Well, the magic of grep has produced two files,

• one file of -ish words
  and
• one file of -like words

Feel free to figure out the rules that they follow.
And to find counterexamples to others' rules.

Solution 3:

Consider: childish vs childlike. Childish gives a passive meaning. She left him because of his childish behaviour. But childlike has a different meaning. It is something like /innocent/ She gave a childlike smile.q