The Statler Brothers – Don't Wait On Me lyrics

It's basically saying when hell freezes over, or when pigs fly. The singer is saying he'll come home to his current paramour whenever a series of impossible (or at least very unlikely) events occurs. In other words, he's not coming back, ever, so don't hold your breath, darlin. Along the way, the singer gets to comment on culture, society, the weather, life in general, and his own personal life by equating the likelihood of certain things to other physically impossible things.
Some clarification may be in order for those born after the 60s. "the charges aren't reversed" refers to a "collect phone call". Before the days of cell phones, prepaid calling cards, and nationwide free calling, when you made a call to someone outside your local landline calling area, someone had to pay some additional "long distance" charges. Normally, that would be the caller. However, if the caller didn't want to pay those charges, he would ask the operator to "reverse the charges". When the call connected the operator would tell the person receiving the call that he had a "collect" call, and ask if he would pay the charges. If not, the call ended there. The singer, basically, is calling his brother-in-law "cheap", because when he calls, it's always collect.
Madlyn o'hair (it's misspelled in the lyrics) was an outspoken atheist, and critic of the catholic church, so it is highly unlikely that the catholic church would ordain her as a priest; doubly so, since the catholic church does not allow female priests.
"load up on a long shot" refers to betting a lot of money on an underdog in a horse race. The odds of that horse winning, computed before the race begins, are 50-to-1 or worse. So if the horse wins, the person who bets on him gets several times the amount of the bet. But such a horse rarely wins, and, apparently, never wins when the singer bets a lot of money on that horse.
The line about wrigley field changed between the original release of song (in the early 70s, I think) and a later release (in the 90s, I think) after the group replaced a retired member. Wrigley field is, of course, the home of baseball's chicago cubs. In the original song, the line was "when the lights come on at wrigley field... ". The chicago cubs, you see, were adamant about never playing night games at home, anmd refused to erect lights in the stadium. Actually, they did plan to install lights in 1942, but the plan was scrapped for the war effort. Perhaps because baseball is so steeped in superstition, it just became a rule that wrigley would never have lights, and the cubs would never play a home game at night. Until 1988, when mlb decided that it would no longer forego the enormous profits of a televised night-time play-off game by allowing the cubs to host it in a stadium with no lights. So mlb laid down the law with the cubs and said, either you put lights on wrigley field, or, if you ever make it to the playoffs, your "home" games will have to be played elsewhere. The cubs folded and wrigley now has lights, and night games (though fewer than most other teams). So, after the group lost and replaced a member, they re-recorded the song, changing the line to "when they put a dome on wrigley field". It is somewhat funny, however, that one of the things that the singer (of the original version) thought to be impossible did, in fact, come to pass. So, I guess, if the woman to whom the song was being sung in the 60s could have just held out for 20-25 years, the singer would eventually come back to her.
Oh, also, though madlyn o'hair was murdered in 1995, and never repented of her atheist beliefs, one of her sons did, in fact, become a baptist minister. I know, it's not exactly the same, but it's still ironic, don't you think? It's also ironic that, after all the death threats she claimed to have received from christians, her murderer was a fellow atheist.