Green’s Dictionary of Slang

(Aus./US) a waste of time; something fated to fail; a disappointment; also attrib.

1905		F. Hutchison Philosophy of Johnny the Gent 87: ‘Cracker sure had him bulled that the skate was a lob’.
1905		F. Hutchison Philosophy of Johnny the Gent 39: ‘Say,’ says I, ‘Skadoo! Back up! You’re on a lob number’.
1914		Sport (Adelaide) 2 Oct. 5/1: They are no judge of music, / They are are not so help me bob, / But get the boys to help you, / Or you are apt to get a lob.
1922		Appleton Post-Crescent (WI) 8 May 11/1: Flapper Dictionary lob – A dumb trick.
1938		D. Runyon ‘All Horse Players Die Broke’ in Runyon on Broadway (1954) 712: Putting the jewellery back [...] he figures he is on a lob.