Actually, the expression is 'pray tell' and it means essentially 'please tell me/us.' It is a piece of rhetoric and uses 'pray' in its archaic meaning of 'implore.' So the expression is a contraction of 'I pray (implore) you to tell me/us.' I hope this helps.

But why, “pray tell” would you ask? Glad you did because many believe it was Shakespeare who coined this phrase. It's a very beautiful old phrase from medieval England and used in the proper context it’s poetic, lively and powerful. Shakespeare’s The Tempest - "Heaven thank you, my dear father," said Miranda "Now pray tell me, sir, your reason for raising this sea-storm?" Shakespeare’s Merchant of Venice also shows the use of the phrase “The thoughts of others! Pray you, tell me this;” Cardinal Wolseley - "Pray, tell 'em thus much from me" Alexander Pope “Pray tell me, sir, whose dog are you?” Before the 13th century, the word was "preien" or to ask earnestly/beg. The old French word "preier" came from the Latin "precari." Prayer also came from the Latin word "prex." From "precari" came precarious which originally meant to be achieved through the asking. Precarious entered the English language in the 17th century. Later we used it as begging a favour in the asking. The usage of "pray" was common in medieval England. It meant please. Earlier writings used "prithee", short for "I pray thee". Example of meaning - "pray tell" "Pray tell but whom dost thou wish to deceive? In the leaving of thee no soul shall grieve." Alatea