dependable adj.

mid-15c., "to be attached to as a condition or cause, be a conditional effect or result," a figurative use, also literal, "to hang, be sustained by being attached to something above;" from Old French dependre, literally "to hang from, hang down," and directly from Latin dependere "to hang from, hang down; be dependent on, be derived," from de "from, down" (see de-) + pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin").

From c. 1500 as "to rely, rest in full confidence or belief;" from 1540s as "be sustained by, be dependent (on)." Related: Depended; depending.

common termination and word-forming element of English adjectives (typically based on verbs) and generally adding a notion of "capable of; allowed; worthy of; requiring; to be ______ed," sometimes "full of, causing," from French -able and directly from Latin -abilis. It is properly -ble, from Latin -bilis (the vowel being generally from the stem ending of the verb being suffixed), and it represents PIE *-tro-, a suffix used to form nouns of instrument, cognate with the second syllables of English rudder and saddle (n.).

A living element in English, used in new formations from either Latin or native words (readable, bearable) and also with nouns (objectionable, peaceable). Sometimes with an active signification (suitable, capable), sometimes of neutral signification (durable, conformable). It has become very elastic in meaning, as in a reliable witness, a playable foul ball, perishable goods. A 17c. writer has cadaverable "mortal."

To take a single example in detail, no-one but a competent philologist can tell whether reasonable comes from the verb or the noun reason, nor whether its original sense was that can be reasoned out, or that can reason, or that can be reasoned with, or that has reason, or that listens to reason, or that is consistent with reason; the ordinary man knows only that it can now mean any of these, & justifiably bases on these & similar facts a generous view of the termination's capabilities; credible meaning for him worthy of credence, why should not reliable & dependable mean worthy of reliance & dependence? [Fowler]

In Latin, -abilis and -ibilis depended on the inflectional vowel of the verb. Hence the variant form -ible in Old French, Spanish, English. In English, -able tends to be used with native (and other non-Latin) words, -ible with words of obvious Latin origin (but there are exceptions). The Latin suffix is not etymologically connected with able, but it long has been popularly associated with it, and this probably has contributed to its vigor as a living suffix.

1560s, raliabill, "that may be relied on, fit to be depended on, trustworthy," originally Scottish; see rely + -able. Not common before 1850, and often execrated thereafter in Britain as an Americanism because it involves a use of -able different from its use in provable, etc., and not warranted in classical Latin. But it is defended (by OED, Century Dictionary, etc.) on grounds of the suffix's sense in available, laughable, livable, dependable; indispensable, etc. Related: Reliably; reliableness. As a noun, "a reliable person, beast, or thing," by 1890 (with old).

Reliable expresses what cannot be expressed by any other one word. Moreover, it conveys an idea of constant occurrence. He who would be exact has, indeed, no alternative, if he avoids it except a periphrasis. [Fitzedward Hall, "On English Adjectives in -Able," 1877]