Definition
A 'lock' in the sports betting community is defined as a 'guaranteed pick'. In most betting circles, calling a pick to be a 'lock' jinxes such a pick.
Usage
Handicappers who sell their picks are often the ones who tout 'locks'. Some give outlandish disclaimers, such as "This is my lock of the century-you will not lose your money", some are even still very outlandish and comical, "This is the lock of my life (part 2)". The sports bettor must realize that a pick can never really be guaranteed. There is always something that can happen, and one should never bet their whole bankroll on any one game. As the old saying goes, ".bet and live another day.the star spangled banner will be played again tomorrow.don't bet the farm today."
Money Management
In terms of money management, a sports gambler should never bet their whole bankroll on a 'lock'. Conventional wisdom dictates that one should never bet any more than 5% of one's bankroll on any one game. If one thinks that a game is a 'lock', then bet the maximum 5% on such a game. This percentage may not sound as much, but if one wants to stay in the game and not be wiped out instantly, one should follow this strategy. Alongside the maximum 5%, each play should be around 2-3% of the bankroll (one unit). A unit is defined as a percentage of a bankroll. The sports gambler should designate a unit value to all his/her plays-this will ensure that all bets are relatively equal. When one is winning, it is alright and even advisable to increase the unit size; conversely, when one is losing it is almost mandatory, if not smart, to decrease the unit size. There are streaks in sports, as in sports gambling-it is up to the sports bettor to realize this and bet accordingly.
Touts
As stated earlier, touts sell their picks and overuse the term 'lock'. Touts are those who sell their picks for a living, and should not be entirely trusted. Most are scam artists and shade their winning records. For example, a well-known handicapper (whom the author will abstain from using his name), once used this tagline, "I am 17-3 over the last month, join on board now and start making some real money!!!". What the handicapper failed to mention is that he was an abysmal 5-20 in the previous month. These 'scamdicappers' are very smart and will try to con an unsuspecting desperate gambler. It is up to the savvy gambler to realize and know that it is almost humanly impossible to be winning (or losing) consistently at a 85% clip.
Summary
In closing, a 'lock' is another term for a guaranteed clip.