Green’s Dictionary of Slang

navvy n.

[SE navvy, a labourer]

SE in slang uses

In compounds

navvy’s piano (n.)

a pneumatic drill.

[UK]Partridge DSUE (8th edn) 782: [...] since ca. 1925.
navvy’s prayer book (n.) [SE prayer-book; the ‘prayerful’ posture the shoveller has to adopt]

a shovel.

[UK]D.W. Barrett Life and Work among Navvies 42: A shovel [...] is generally spoken of as ‘the Navvy’s Prayer-Book’.