From where did the RC practice of not eating meat on Fridays come, especially during Lent?

I think its obvious...from big friday of course ...

Catholics abstain from meat on Fridays because Jesus died for our sins on Good Friday, and it's a way of mourning and doing penance for the sins for which Christ died. (By the same token, Sunday-the day on which Christ rose for our salvation-is an especially appropriate day to rejoice.) It's meat because it should be a sacrifice - giving up something that we enjoy and in the past meat was a luxury of people - so they are giving up a luxury.

http://www.cafemom.com/answers/138555/lent_no_meat_on_fridays

In Western Christianity , Lent begins on Ash Wednesday and concludes on Holy Saturday. The six Sundays in Lent are not counted among the forty days because each Sunday represents a "mini-Easter", a celebration of Jesus' victory over sin and death.

https://en.wikipedia.org/wiki/Lent

those churches which follow the Byzantine tradition (e.g. Eastern Orthodox and Eastern Catholics)

The Roman Catholic Church believes that all people are obliged by God to perform some penance for their sins
https://en.wikipedia.org/wiki/Fasting_and_abstinence_in_the_Roman_Catholic_Church
This has changed over time to many variations most notably after Vatican II, Prior it was all Fridays + fasting from midnight prior to receiving Eucharist.
It should be mentioned that the early Popes that set the Canon laws where Fishermen and you where encouraged to eat fish once per week. This is unsubstantiated and just a commonly held belief.

The Talmud discusses various customs in Judaism regarding meat. One custom, which is not widely kept among Ashkenazi Jews today, is to avoid eating meat on Friday before the sun goes down, to save the meat for the Sabbath. (Some people don't eat at all on Fridays, to make the Friday night meal more festive.) The reason for this is that most Jews consider it a requirement to celebrate the Sabbath with a meat meal (Numbers 28 calls for extra sacrifices on the Sabbath), and in the day when meat was not as easy to come by and had to be conserved, this would mean not eating it on the day before the Sabbath - Friday. It appears that the Church picked up the custom, gave it a more celebratory Christian meaning (which is not uncommon, the whole idea of Lent evolved from Jewish custom), and that simply became better known.

Just for interest sake. It is NOT just Fridays during Lent it is still technically every Friday

"Most Catholics think that Vatican II did away with the requirement of not eating meat on any Friday of the year. Most think it is now just Ash Wednesday and the Fridays of Lent that we cannot eat meat.

This is what the new Code of Canon Law brought out in 1983 says about the matter:

Canon 1251
Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday.
Canon Law still requires that Catholics not eat meat on Fridays!

Of course, most Episcopal Conferences have determined that, instead of abstaining from meat, Catholics may perform an act of penance of their choosing. "

http://www.catholic-pages.com/life/fridaymeat.asp

But don't worry, if you do eat meat on a Friday (even Good Friday) it is not a sin.

http://wiki.answers.com/Q/Why_are_Catholics_not_allowed_to_eat_meat_on_Fridays_during_Lent