Question about words ending with '-dle'

Solution 1:

The suffix you are looking for is -le, not -dle

According to dictionary.com:

1. a suffix of verbs having a frequentative force: dazzle; twinkle.

2. a suffix of adjectives formed originally on verbal stems and having the sense of “apt to”: brittle.

3. a noun suffix having originally a diminutive meaning: bramble.

4. a noun suffix indicating agent or instrument: beadle; bridle; thimble.

Solution 2:

The suffix is actually -le or -el (which represent a variation in spelling, the -le being the more common spelling in modern English). Consult etymology online. The preceding consonant, as in your words is often a d, nd,...

See also the Oxford English Dictionary, which gives three different derivations of -le, including

1 The usual modern English form of Middle English -el(e, -le, representing Old English -el, -ela, -(e)le in nouns and -ol, -ul, -el in adjectives.

The Old English nouns and adjectives with l suffixes are probably in most cases of pre-English formation. The nouns formed on noun-stems have sometimes an originally diminutive sense, as in bramble; sometimes they express the notion of ‘an appliance or tool’, as in thimble, handle. In those formed on verb-stems the function of the suffix is either agentive as in beadle, instrumental as in bridle, girdle, or expressive of some less definable relation, as in bundle. The adjectives, which are formed on verb-stems, have the sense ‘apt or liable’ (to do what the verb expresses), as in brittle, fickle, gripple, nimble, †swikel.

By pre-English formation, it means the suffix was already part of the word when it entered English, whether from Old German, Old Dutch, even Latin (example, fiddle may derive from Latin vitula , vidula, whereas handle comes from Old German).

Note also

The form -el (suffix1) is retained where phonetic law or orthographical convention does not permit the change into -le, as after ch, g soft, n, r, sh, th, and v. After m the suffix becomes -ble.

Quotes from Oxford English Dictionary, emphasis mine; abbreviations spelled out.