Tmesis comes from the Greek word meaning a 'cutting'. The term describes when one word (or phrase) cuts into another word. (In some definitions the word being cut into should be a compound word - so that there is a natural point for the incision. It is usually used for comic effect, but not always, as the examples show:
From Shakespeare's Romeo and Juliet:
This is not Romeo; he is some other where.
From Troilus and Cressida:
A strange fellow here
Writes me that man—how dearly ever parted...
From Richard II:
If on the first, how heinous e'er it be,
To win thy after-love I pardon thee.
From Donne's Hymn to Christ:
Whatseasoever swallow me, that flood
Shall be to me an emblem of thy blood.