As the attrition and casualty rates rose during the Spring offensive, it became standard practise amongst Guard regiments in the Second and Third armies to keep a core of soldiers – typically two members from each squad and either the CO or second-in-command – back from major offensives. This was termed LOBO, or 'Left Out of Battle Order'.
If the bulk of the force was destroyed, ad hoc companies made of whiteshields and volunteers could be built up around the LOBO officers and trooperstroopers; helping to accelerate the deployment of battle-ready forces.
Corp Ynbar was well known for his gung-ho attitude, and refused to become LOBO. Impressing the Commissars with an impassioned (if not particularly eloquent) argument for his frontline place, Ynbar was forwarded with his squad. His squadmates, displaying the typical crude humour of the Lamb's Worlders, daubed his chestplate with his new nickname.