impeachable adj.

formerly also empeach, late 14c., empechen, "to impede, hinder, prevent;" early 15c., "cause to be stuck, run (a ship) aground," also "prevent (from doing something)," from Anglo-French empecher, Old French empeechier "to hinder, stop, impede; capture, trap, ensnare" (12c., Modern French empêcher), from Late Latin impedicare "to fetter, catch, entangle," from assimilated form of in- "into, in" (from PIE root *en "in") + Latin pedica "a shackle, fetter," from pes (genitive pedis) "foot" (from PIE root *ped- "foot").

In law, at first in a broad sense, "to accuse, bring charges against" from late 14c.; more specifically, of the king or the House of Commons, "to bring formal accusation of treason or other high crime against (someone)" from mid-15c.  The sense of "accuse a public officer of misconduct" had emerged from this by 1560s. The sense shift is perhaps via Medieval Latin confusion of impedicare with Latin impetere "attack, accuse" (see impetus), which is from the Latin verb petere "aim for, rush at" (from PIE root *pet- "to rush, to fly").

The Middle English verb apechen, probably from an Anglo-French variant of the source of impeach, was used from early 14c. in the sense "to accuse (someone), to charge (someone with an offense)." Related: Impeached; impeaching.

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.

1784, from un- (1) "not" + impeachable. Related: Unimpeachably.