adverb, adjective Nautical.
as slow as possible without losing steerageway.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for dead slow
Historical Examples of dead slow
They've stolen my ship from me, and I can tell by the roll of her she's practically hove to under a dead-slow bell this minute.Cappy Ricks Retires
Peter B. Kyne