primary meaning of ‘boggle’ was ‘to start with fright’, originally with reference to horses—probably related to the nouns ‘bogle’ and ‘bogey’, denoting an evil spirit such as horses are reputed to see
The image of the deaf adder originated in the Book of Psalms, 58:4-5: “the deaf adder that stoppeth her ear; which will not hearken to the voice of charmers”.
‘To eavesdrop’ originally referred to standing within the eavesdrop (the ground on to which water drips from the eaves of a house) in order to overhear what is going on inside.