If someone gave you a slap in a dream, then it means that you feel neglected and not taken for granted by someone. If you gave a slap to somebody, it shows the anger and frustration you have towards that particular person.
Dream dictionary: Slap dream meanings
…Dreaming that you’re receiving a slap indicates negligence. Dreaming that you’re giving a slap suggest that you’re suppressing some deep anger and repressed rage….
If we give one, it portends people will be unfair to us. If someone gives us one, it tells us we will receive a punishment for our wickedness.
Give a kiss on the cheek indicates pure feelings. Slap or receive a slap is a sign of resentment in the family.
…(Blow | Criticize | Smack) To lightly slap someone in a dream means to reprimand him, or to oblige him to recognize a favor, boasting about one’s generosity, or constantly reminding the other person of some shabby assistance. If one playfully slaps someone lightly on his cheek in a dream, it also means that the latter owes him something or a favor and the opposite is interpreted accordingly. Slapping someone’s face with anger in a dream means that the victim will triumph over the assailant. To slap someone in a dream also means recognition of his seniority or superiority. Threatening someone in a dream also maybe a recognition of his superiority. (Also see Digging up the past | Slapping | Threat)…
…(Blow | Pound | Shake | slap) In a dream, pounding one’s head, or smiting one’s face means begetting a son at an old age….