Twas' is a contraction of the phrase 'It was, ' which is often used in poetry and literature to begin a story or poem. The term 'Brillig, and the slithy toves' is a line from Lewis Carroll's poem 'JABBERWOCKY, ' which appears in his novel Through the Looking-Glass. The poem is about a creature called the Jabberwock, and the line is meant to describe the strange creatures that inhabit the world of the poem.