The Collaborative International Dictionary of English v.0.48:

Sermon \Ser"mon\, n. [OE. sermoun, sermun, F. sermon, fr. L.
   sermo, -onis, a speaking, discourse, probably fr. serer,
   sertum, to join, connect; hence, a connected speech. See
   Series.]
   1. A discourse or address; a talk; a writing; as, the sermons
      of Chaucer. [Obs.] --Chaucer.
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   2. Specifically, a discourse delivered in public, usually by
      a clergyman, for the purpose of religious instruction and
      grounded on some text or passage of Scripture.
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            This our life exempt from public haunts
            Finds tongues in trees, books in the running brooks,
            Sermons in stones and good in everything. --Shak.
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            His preaching much, but more his practice, wrought,
            A living sermon of the truths he taught. --Dryden.
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   3. Hence, a serious address; a lecture on one's conduct or
      duty; an exhortation or reproof; a homily; -- often in a
      depreciatory sense.
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The Collaborative International Dictionary of English v.0.48:

Sermon \Ser"mon\, v. i. [Cf. OF. sermoner, F. sermonner to
   lecture one.]
   To speak; to discourse; to compose or deliver a sermon.
   [Obs.] --Holinshed.
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         What needeth it to sermon of it more?    --Chaucer.
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The Collaborative International Dictionary of English v.0.48:

Sermon \Ser"mon\, v. t.
   1. To discourse to or of, as in a sermon. [Obs.] --Spenser.
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   2. To tutor; to lecture. [Poetic] --Shak.
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WordNet (r) 3.0 (2006):

sermon
    n 1: an address of a religious nature (usually delivered during
         a church service) [syn: sermon, discourse, preaching]
    2: a moralistic rebuke; "your preaching is wasted on him" [syn:
       sermon, preaching]