Definition of an heir and a spare in English:

an heir and a spare

phrase

British
humorous
  • Two children (typically in reference to members of the monarchy or nobility, said to need two children, one to succeed to a title and the other to guarantee the family line should anything happen to the first)

    ‘the future King wants to follow in his parents' footsteps by producing the minimum royal issue of an heir and a spare’
    • ‘She did have two kids with Charles - an heir and a spare.’
    • ‘Her job, just like others before her, is to produce an heir and a spare.’
    • ‘Although about half have provided an heir and a spare to inherit their fortunes, almost half have not even seen the inside of a maternity ward.’
    • ‘He had the requisite heir and a spare but unfortunately all three had died in a boating accident.’
    • ‘She was purely there to produce an heir and a spare.’
    • ‘In those days of high infant mortality, it was not enough to have one son, or even an 'heir and a spare' as the British say.’
    • ‘The latest edition to the heir and a spare Royal rank, the baby will replace Prince Harry as fourth in line to the throne.’