An unprofitable or futile message. The Saxon bot means “reparation”- “overplus to profit”; as “I will give you that to boot”; “what boots it me?” (what does it profit me?).
I sent him Bootless home and weather-beaten back.
Shakespeare: 1 Henry IV., iii. 1.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894