From mother +‎ -er.

motherer (plural motherers)

1. Agent noun of mother: one who mothers.
   • 1966, Frank Swinnerton, Sanctuary, page 165:

     ...the hag-like Rose, abrupt flouter of authority and motherer of naughty old girls...

   • 1985 February 2, Tim Broadbent, “Kindred Spirits (review)”, in Gay Community News, volume 12, number 28, page 10:

     A sequel to The Peter Pan Syndrome (about men who refuse to grow up), this book is filled with information for women who find that they are compulsive motherers.

   • 2009, Ursula K. Le Guin, Powers, page 34:

     Our aunties said I didn't need a motherer, since I had such a good sister, and I agreed.