"O what can ail thee, knight-at-arms"

Here is a poem written by the English poet John Keats (1795-1821).

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"O what can ail thee, knight-at-arms,
  Alone and palely loitering?
The sedge has wither'd from the lake,
  And no birds sing.

"O what can ail thee, knight-at-arms!  
  So haggard and so woe-begone?
The squirrel's granary is full,
  And the harvest's done. (1-8)
. . .
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This poem is "La Belle Dame sans Merci" ("The Beautiful Lady Without Mercy"). The story features a dying knight who was "ailed" by an encounter with a mysterious lady.

The use of "ail" in the first line of each stanza is interesting. In present English, the verb is not used in everyday conversation, far from being "colloquial." So people don't usually say, "You don't look well today, what's ailing you?"

Going back in time,

In Old/Middle English, "ail" was widely used in the sense of "causing trouble or harm." It is a word typically inherited from Germanic as the one syllable structure indicates.

Usually, the basic words of daily use tend to remain common up to the present time. For example, “go, eat, ask” existed since the beginning of the English history (the arrival of the Anglo-Saxon, an ancestor of the English).

It is often noted that the English lexicon consists of groups with three etymological levels, whose usage differs according to the context. For example, "ask" is a native lexicon (Old English). English has the word equivalent to its meaning, "question," a newcomer from French, which produces more cultural atmosphere. There is also "interrogate," a word from Latin, which emits a technical, pedantic tone.

These distinctions stem from the multilingual history in medieval Britain, where the repeated invasion took place and the power of domination shifted. The historical perspective into English helps understand the nuances each word carries.

Back to "ail," if "ask" is a kind of chatty word, one may wonder what happens to "ail" in Keat’s poem. Surely, the word serves as a "poetic diction," and appeals to an "archaic" aura it gives. But the question remains why “ail” has retained “archaic” quality up until then (and now) despite the fact that the word has been firmly rooted in the native soil. 

It is curious to think how a native word has lost its secular touch and become less common in a informal sphere. It requires an extensive research.