A novel observable measuring the $C!P$ asymmetry in multi-body decays of heavy mesons, which is called the forward-backward asymmetry induced $C!P$ asymmetry (FBI-$C!P$A), $A_{CP}^{FB}$, is introduced. This observable has the dual advantages that 1) it can isolate the $C!P$ asymmetry associated with the interference of the $S$- and $P$-wave amplitude from that associated with the $S$- or $P$-wave amplitude alone; 2) it can effectively almost double the statistics comparing to the conventionally defined regional $C!P$ asymmetry. We also suggest to perform the measurements of FBI-$C!P$A in some three-body decay channels of charm and beauty mesons.
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