SC+thrash

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 * Thrash:**
 * to beat soundly in punishment; flog.
 * to make way against the wind, tide, etc.; beat.

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Philip Booth, the author of the poem First Lesson, is connecting the word thrash in the sentence "When you tire on the long thrash to your island." He was trying to say that overtime children swim from the sea and make their way to the huge ocean, and over the period of time they develop into adulthood. The "island" would be the ocean in this case. ======