This paper develops a linguistically robust encryption, LUNABEL, which converts a message into semantically innocuous text. Drawing upon linguistic criteria, LUNABEL uses word replacement, with substitution classes based on traditional word replacement features (syntactic categories and sub-categories), as well as features under-exploited in earlier works: semantic criteria, graphotactic structure, inflectional class and frequency statistics. The original message is further hidden through the use of cover texts — within these, LUNABEL retains all function words and targets specific classes of content words for replacement, creating text which preserves the syntactic structure and semantic context of the original cover text. LUNABEL takes advantage of cover text styles which are not expected to be necessarily comprehensible to the general public, making any semantic anomalies more opaque. This line of work has the promise of creating encrypted texts which are less detectable than earlier steganographic efforts.