"Once bitten twice shy" is an idiom that is derived from an old English proverb. The general intent of the saying is to convey the concept that once an individual has been hurt in some manner, he or she is likely to be very cautious of any situation that contains elements of that previous bad experience. A similar saying, "once burned twice shy," conveys the same general sentiment, noting the caution that people will employ when facing a similar situation.
The imagery associated with the term usually conjures up a scene in which someone sustains a dog bite or a snake bite. Both types of bites can be extremely painful and leave emotional as well as physical scars. As a result, the injured party may be extremely cautious when around a dog or snake in the future, taking care to not do anything that would prompt another bite.
In general usage, "once bitten twice shy" can be applied to a number of scenarios. When referring to a romantic situation, someone who was deeply hurt in a previous relationship may be very reluctant to enter into a new one. This is especially true if the new person of interest exhibits certain qualities or habits that remind the injured party of that previously failed relationship. As a result, opportunities to start a relationship with someone new may be few and far between, until that old hurt is healed sufficiently that it no longer affects the perception of new potential partners.
At work, employees may be careful when interacting with a coworker who has caused difficulties in the past. Due to the past history, it may be considered wise to refrain from discussing certain matters or sharing information that is not directly relevant to carrying out some assigned task. Only once the co-worker has demonstrated his or her ability to not repeat the actions that caused problems in the past will others begin to let down their guard and be more accepting of that individual.
Generally, someone who is "once bitten twice shy" feels the need to move forward with a great deal of caution, at least until any fears surrounding that movement have been addressed and resolved. A natural predilection of human nature, it boils down to personal preservation, avoiding injury and hurt by taking into account what has happened in the past and using that information to avoid a repetition of undesirable events. In this manner, just about everyone has lived the concept, even if they have not actually used those words.