Green’s Dictionary of Slang

souper n.1

[such conversions, however nominal, were often achieved by the appeal of Protestant missionaries handing out free soup, orig. at the time of the great famine of 1845–7]

1. [mid-19C+] (orig. Irish) a convert from Roman Catholicism to Protestantism; thus any Protestant; thus souping n., conversion fron Roman Catholicism to Protestantism.

2. [late 19C] one who scrounges free soup tickets.