Motivated by searches for 0 ν β β decay in nuclear experiments and collider probes of lepton number violation at dimension d ≥7 , we investigate the sensitivity to the d =5 Weinberg operator using the nonresonant signature p p →ℓ±ℓ'±jj at the LHC. We develop a prescription for the operator that is applicable in collisions and decays, and focus on the ℓℓ'=μ μ channel, which is beyond the reach of nuclear decays. For a Wilson coefficient C5μ μ=1 , scales as heavy as Λ ∼8.3 (11 ) TeV can be probed with L =300 fb-1(3 ab-1) . This translates to an effective μ μ Majorana mass of |mμ μ|∼7.3 (5.4 ) GeV and establishes a road map for testing the Weinberg operator at accelerators.